HCA: Hermit Crab Association

I have a couple of UTHs (under-tank heaters), "Zoo Med" brand, by Repti-Therm, attached to the sides of my 20 gal tank. One looks like about 6" long, the other is hard to see at the moment but it's the next size up (maybe 8 or 10 inches long). The smaller one is pretty old and has been used on another tank and doesn't stick terribly well, although I've had it on the current tank for a couple years and I keep it up against the glass by propping a metal spray can up next to it. The other one was brand new when I got it a couple years ago, but even so, it also has trouble sticking - maybe half of it adheres to the tank. It's on the back side so I have no solution other than pushing it with my hand every once in a while (in other words, no real solution). I recognize that the position and tension of the power cord plays a significant role in the forces applied to the UTH, but even messing with those does not solve the problem.

I'm wondering what the solutions are. Would it be safe to use duct tape along the edges that are peeling? Or perhaps there is a better brand? Or some kind of stickum to help them adhere to the glass?

These types of UTH's are not meant to be removed and re-stuck. Doing so can be a fire hazard. So I would not be re-using the one you peeled off. If the other one has not been removed and re-stuck, I would agree with taping the edges of it.

UTH's from Reptile Basics, and Flukers are better brands than the ZooMed ones. They do not have a sticky side, and are attached with tape.

I think the small one (that was on a different tank) basically unpeeled itself -- in other words I didn't have to distort it to remove it, so it seems very unlikely it was damaged. But, thanks for the recommendations, I'll look for those higher quality ones you mentioned. Tonight, I did try adding some black electrical tape. It didn't help much with the larger one, which overpowers the tape when it starts peeling/warping, despite taping a few inches of the power cord as well. The tape was more helpful for the little one. I used the tape mostly on the power cord, since that clearly poses no risk of excess heat buildup, and given the small size the cord is relatively more important in terms of mechanical support.